On Oct 4, 2009, at 3:24 PM, Jerry Flanders wrote:
> I used FSK for years but my present radio does not allow it
> and I find no downside to using AFSK so far. The receiving station
> cannot even tell what mode you are using, so there is no difference
> at all on the receiving end.
Jerry is perfectly correct.
The only downside of "AFSK" is that you need to carefully maintain the
audio output from your sound card so as not to overdrive the
transmitter.
If you are using the microphone input of the rig, extra care needs to
be taken since there is a possibility that you can overdrive the
microphone preamp without overdriving the modulator. But most rigs
have a special AFSK/PKT/Digital/whatever-they-want-to-call-them input
that bypasses both the microphone preamp and any audio processing.
In addition to not using audio processing, you need to make sure that
the PA stage of the transmitter has no ALC applied to it. Some rigs
that use DSP modulators abuse the term "ALC" and they can show ALC on
the meter and yet the PA stage is working perfectly linearly. So,
check with the manual as to whether you can allow ALC to show up on
the rig's front panel meter when using AFSK.
Once you take care of these items, AFSK signals can be cleaner than
FSK signals.
If you are using a fixed tone pair (i.e., if you are not clicking on a
waterfall to tune around), you only have to make the adjustment once,
and occasionally checking if the settings remain OK. If you are using
frequency agile AFSK, you need to make sure the frequency response of
passband that you are transmitting AFSK in is sufficiently flat so
that you won't over-modulate when certain tone pairs are selected.
Some software modems have implemented audio output equalizers for you
to compensate for a transmitter passband that is not flat. Check with
the software documentation.
If you are going to use a fixed tone pair, it is better to select a
high tone pair (2125 Hz/2295 Hz) since in the event of over-
modulation, your harmonics will fall outside of the transmitting
modulator's output filter, and cause much less harm, if any, depending
on the quality of the transmitting filter.
Furthermore, many rigs today have built in VOX for AFSK modes. You
don't have to lift a finger to address the PTT issue when it comes to
transmitting 100% duty cycle modes such as RTTY and MFSK modes (e.g.,
Olivia and DominoEX), and VOX usually work fine too for almost-100%
duty cycle codes such as PSK31.
And guess what? You will not be puzzled when people tell you that
your FSK signal is inverted or reversed.
With all the rigs and digital interface combinations that you find out
there, there is a 50-50 chance that you are inverted when you start
out using FSK. I have seen long-time RTTY users who need to be
prodded into transmitting a normal signal instead of inverted (mark
and space carriers swapped) signal. Often, you will find inverted
signals calling in a pileup of a rare DX station; those poor saps
didn't even know that they are inverted and no one will bother to
switch over to copy them.
Pretty much every AFSK software is set up such that if you can copy a
signal, your transmit signal will be in the same polarity unless you
explicitly ask the software to invert the RTTY signal. So there is
less here too that can go wrong.
73
Chen, W7AY
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